Inversion tectonics at the Northern Margin of the Paris Basin (northern France): new evidence from seismic profiles and boreholes interpolation in the Artois Area
Résumé
A synthesis of existing borehole data and seismic profiles has been conducted in the Artois area (northern France), along the northern border of the Paris basin, in order to explore the possible control exerted at depth by the Upper Carboniferous Variscan thrust front on the distribution of Late Paleozoic-Mesozoic depositional centers and their subsequent uplift in Tertiary times. Such control was demonstrated recently in the Weald-Boulonnais basin (Eastern Channel area) that forms the western prolongation of the area under study but was so far poorly constrained in the Artois area. Presented data provide evidence for the topography of the Artois hills and the altitude of sedimentary layers to be controlled by the activity of a network of relaying WNW-ESE striking faults inducing the systematic uplift of the southern fault blocks. Those steeply S-dipping faults branch downward onto the ramp of the Variscan thrusts forming lis tric faults that locally Iimit to the north buried half-graben structures, filled with fan-shaped fluviatile Stephanian-Permian deposits. Such c1ear syn-rift geometry shows that the ramp of the main Variscan frontal thrust (the Midi thrust) has been reactivated as a normal fault in Stephanian-Permian times thus forming a very demonstrative example of a negative inversion process. The reverse offset of the transgressive Middle Cretaceous-Lower Eocene layers covering unconformably the Paleozoic substratum argue for a Tertiary (Middle Eocene-Late Oligocene?) contractional reactivation of the fault network thereby documenting a repeated inversion process along the Artois Variscan thrust front. The Variscan frontal thrust zone is thus shown here to represent a prominent crustal-scale mechanical discontinuity that localized deformation in the Artois-Boulonnais area since Upper Paleozoic times.